Some of the most notorious alien sighting have been explained away by scientists but mystery surrounds others:

1. 1947 Roswell crash: UFO proponents claimed that the US military had captured a crashed alien aircraft. This well-publicised, controversial incident became a pop culture phenomenon.

Explanation: the US military maintained that it had recovered debris from an experimental high-altitude surveillance balloon belonging to a classified programme named “Mogul”.

2. 1947 Kenneth Arnold case: the press coined the term “flying saucer” after this American businessman and pilot claimed he had seen nine objects flying in a chain near Mount Rainier, Washington. Arnold described them as saucers skipping across water.

3. 1952 Washington, D.C. flap: this series of UFO reports was accompanied by radar contacts at three separate airports. Country-wide headlines spurred the formation of the CIA Robertson Panel.

Explanation: the US Air Force suggested that a temperature inversion - in which a layer of warm, moist air covered a layer of cool, dry air closer to the ground - had caused radar signals to bend and give false returns.

4. 1957 Levelland case: police investigated numerous motorist reports of engines stalling when encountering a glowing, egg-shaped object. Motorists claimed that their vehicles had restarted after the "object" had left.

Explanation: an air force investigation concluded that an electrical storm had caused the sightings and vehicle failures.

5. 1966 Westall encounter: more than 200 students and teachers at two schools in Melbourne allegedly saw a UFO that descended into a grass field. The object then ascended over a local suburb, according to reports. Witnesses still gather for reunions.

Explanation: Australian Skeptics, a non-profit organisation which investigates paranormal and pseudo-scientific claims by using scientific methodologies, believed that the object was an experimental military aircraft.

Explanation: The Canadian Department of National Defence officially classified this sighting as unsolved following a naval search and investigation. The Condon Committee, which investigated UFOs at the University of Colorado, failed to resolve the case.

7. 1976 Tehran incident: A UFO was believed to have disabled the electronic equipment of two F-4 interceptor aircraft as well as ground control equipment. The Iranian generals involved said on public record that they had thought the object was extraterrestrial.

Explanation: UFOs: The Public Deceived, a book by Philip Klass, claimed that witnesses saw an astronomical body - probably Jupiter - and pilot incompetence and equipment malfunction accounted for the rest.

8. 1986 São Paulo chase: around 20 UFOs were seen and detected by radar in various parts of Brazil. They reportedly disappeared as five military aircraft were sent to intercept them.

Explanation: Geoffrey Perry, a British space researcher, attributed the incident to debris that were ejected by Soviet space station Salyut-7 and re-entered Earth’s atmosphere around central-western Brazil.

9. 1989/1990 Belgium wave: around 13,500 people claimed to have witnessed large, silent, low-flying black triangles. Around 2,600 filed written statements describing what they had seen. The frequently-photographed wave was tracked by NATO radar and jet interceptors and investigated by Belgium’s military.

Explanation: Renaud Leclet, a French ufologist, believed some of the sightings could have been explained by helicopters.

10. 2008 Turkey video: a night guard at the Yeni Kent Compound claimed he had videotaped multiple UFOs over a period of four months. Reported witness confirmations spurred claims by Sirius UFO Space Science Research Center it was the “most important images of a UFO ever filmed”.

Explanation: Turkish scientists claimed it was a computer-animated hoax.